Les roches du montagnard
Grès et calcaires
pour une escalade
tout en finesse
Granits et gneiss
pour une escalade
un peu plus athlétique
Schistes
pour se faire peur
Le grès
Est constitué de grains de sable liés entre eux par un ciment. Il est, de par sa nature même, friable. Mais s’il est recouvert d’une patine qui le met à l’abri de la désagrégation on a alors une roche compacte et solide. La surface peut en être relativement lisse avec des grattons allongés (réglettes), des trous et des excavations, là où l’érosion éolienne s’est exercée. On le trouve rarement en haute montagne. En France, la forêt de Fontainebleau est connue pour ses blocs de grès. Annot, au coeur des Alpes de Haute-Provence, offre de magnifiques falaises de grès.
Les calcaires
Forment une vaste famille dont les membres se rencontrent partout. Des Calanques à l’Annapurna, les calcaires s’entassent sur 8 kilomètres d’épaisseur et servent d’armature à la plupart de nos montagnes. En Europe, on le trouve en général à moins de 3000 mètres, sauf dans l’Oberland Bernois, les Dolomites et la Vanoise. C’est une roche à base de carbonate de chaux.
Certains calcaires contiennent des débris organiques d’autres, comme les coraux, ont étés construit par des organismes vivants, c’est le cas du calcaire urgonien du Vercors ou de la Chartreuse. Certains contiennent de la silice, d’autres du carbonate de magnésium (dolomie).
Le caractère primordial du calcaire est sa solubilité. Selon sa composition, le calcaire engendre des formes très diverses. Dans le cas de la dolomie, par exemple, le carbonate de chaux est soluble tandis que le carbonate de magnésium ne l’est pas d’où les formes ruiniformes, très travaillées du relief dolomitique. C’est une roche qui est également sensible au gel. Le résultat, ce sont les chutes de pierres incessantes de l’Eiger et de la Grande Casse.
Au total le calcaire n’est pas sans défauts, mais son abondance et sa variété en font la roche reine du varappeur. Il engendre un type d’escalade aérienne, tout en finesse. C’est la roche pédagogique par excellence: « tout dans la tête et les pieds ».
Les granits
Sont la roche mère de l’écorce terrestre. Ils n’apparaissent pas souvent à la surface dans les massifs jeunes. Les granits sont relativement importants dans les Pyrénées et en Corse. Dans les Alpes, on les trouve surtout dans la zone axiale des Alpes occidentales: massif des Ecrins, chaîne du Mont-SBlanc, en Suisse centrale (Salbitschin), et dans les Grisons (Piz Badile).
C’est une roche à structure purement cristalline qui évolue par altération chimique. Cela donne une surface extrêmement rugueuse, très adhérente.
La Protogine est un granit dont les cristaux ont été déformés par compression et dont le mica a été chloritisé. Il est très solide et donne de superbes monolithes tels que les Drus, le Capucin et la Dibona. Ce granit présente des diaclases qui favorisent l’érosion mécanique. La roche tend à s’écailler en fragments de grande taille, c’est la desquamation. Peu plastiques, ces granits cassent selon des lignes nettes et engendrent de belles failles.
Ces structures déterminent un style d’escalade particulier avec prépondérance des mouvements d’opposition et des coincements.
Le gneiss
A la même composition minérale que le granit (quartz, mica, feldspath), mais avec une disposition stratifiée des éléments. C’est une roche multicolore avec des effets de gaufrage.
Il y a différents types de gneiss. Le gneiss piémontais est comparable au meilleur des granits avec toutefois une rugosité moindre, une grande fréquence de petites écailles et une forte sculpture superficielle des dalles (Argentera, Grand Paradis, Aiguille du Pouce). Le gneiss de l’Oisans lui, chevauche les granits dans presque tous les grands sommets du massif homonyme. Il se caractérise par une grande raideur, par une structure imbriquée riche en prises mais souvent difficile à pitonner. Les gneiss helvétiques et tyroliens se retrouvent dans la plupart des massifs des Alpes centrales (Cervin, Oberland). Il se signale par des constructions pyramidales grandioses. Il est souvent fracturé et pas toujours solide. On pourrait le situer quelque part entre le gneiss piémontais et les schistes cristallins. Il donne de belles arêtes.
Pour gravir le granit et le gneiss « En plus de la tête et des pieds… il faut avoir des bras ».
Les schistes
Il présentent une structure très feuilletée. Les pentes sont rarement fortes. Ce sont des roches à gravir … par les couloirs de glace. Les schistes cristallins se présentent sous forme de plaques solides et compactes aux rebords friables. Ils donnent de belles dalles aux arêtes tranchantes avoisinant de lamentables éboulis.
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